Add JSON-RPC capability

Inline nlohmann/json for the JSON parsing itself, and handle requests
with the same SCGI interface as XML-RPC.

Based off the work in https://github.com/jesec/rtorrent

src/rpc/jsonrpc.h

@@ -0,0 +1,24 @@
+#ifndef RTORRENT_RPC_JSONRPC_H
+#define RTORRENT_RPC_JSONRPC_H
+
+#include <functional>
+
+#include <cstdint>
+
+namespace rpc {
+
+class JsonRpc {
+public:
+  using slot_write = std::function<bool(const char*, uint32_t)>;
+
+  void initialize() {};
+  void cleanup() {};
+
+  bool process(const char* in_buffer, uint32_t length, slot_write callback);
+
+  void insert_command(const char* name, const char* parm, const char* doc) {};
+};
+
+} // namespace rpc
+
+#endif